[PATCH] Arm64: convert soft_restart() to assembly code

[Mark Rutland]

Hi Geoff,

On Fri, Aug 15, 2014 at 07:53:19PM +0100, Geoff Levand wrote:
> Hi Mark,
> 
> On Fri, 2014-08-15 at 19:21 +0100, Mark Rutland wrote:
> > On Fri, Aug 15, 2014 at 06:20:21PM +0100, Geoff Levand wrote:
> > > For the cpu-ops shutdown I'm working on I need a call to move the
> > > secondary processors to an identity mapped spin loop after the identity
> > > map is enabled.  I want to do this in C code, so it needs to happen
> > > after the identity map is enabled, and before the dcache is disabled.
> > > 
> > > I think to do this we can keep the existing soft_restart(addr) routine
> > > with something like this:
> > > 
> > > void soft_restart(unsigned long addr)
> > > {
> > > 	setup_mm_for_reboot();
> > > 
> > > #if defined(CONFIG_SMP)
> > > 	smp_secondary_shutdown();
> > > #endif
> > > 
> > > 	cpu_soft_restart(addr);
> > > 
> > > 	/* Should never get here */
> > > 	BUG();
> > > }
> > > 
> > 
> > I don't follow why you need a hook in the middle of soft_restart. That
> > sounds like a layering violation to me.
> > 
> > I assume this is for implementing the spin-table cpu-return-addr idea?
> 
> Yes.
> 
> > If so, what's wrong with something like:
> 
> > void spin_table_cpu_die(unsigned int cpu)
> > {
> > 	unsigned long release_addr = per_cpu(return_addr, cpu);
> > 	
> > 	/*
> > 	 * We should have a local_disable(DBG|ASYNC|FIQ|IRQ) function or
> > 	 * something similar as these are all context synchronising and
> > 	 * therefore expensive.
> > 	 */
> > 	local_dbg_disable();
> > 	local_async_disable();
> > 	local_fiq_disable();
> > 	arch_local_irq_disable();
> > 
> > 	soft_restart(release_addr);
> > }
> 
> OK, this is a much simpler way than what I was thinking, which
> was to have the secondaries spin in the kernel until the main
> cpu shutdown.  I'll switch over to this, thanks.

I just realised that this is still missing the jump to EL2 that I
mentioned a while back.

I think what we need to do is:

* Have KVM (if present) tears itself down prior to cpu_die, restoring
  the __hyp_stub_vectors in VBAR_EL2 and disabling the MMU, and caches.
  
* Add a mechanism to __hyp_stub_vectors to allow a hypercall to
  call a function at EL2. We should be able to replace the current
  hyp_stub el1_sync handler with that, and rework KVM to call a function
  at EL2 to setup VBAR_EL2 appropriately at init time.

* Depending on whether EL2 is available, go via soft_restart or the
  hypercall to cpu_soft_restart (or something very close to it).

How does that sound?

Mark.